1. Field of the Invention
The present invention relates to an article storage house, i.e. an article storage house in a clean room which temporarily stores semiconductor wafers, crystal display substrates, reticles, disks, etc. in a clean room.
2. Prior Art
There is an article storage house in a clean room as disclosed in Japanese Patent Laid-Open Publication No. 1-41561. The article storage house has an article storage chamber, and article storage shelves provided in the article storage chamber and having a plurality of sectioned accommodating spaces which are arranged vertically and laterally and a carry-in/out unit having a carry-in/out mechanism which can freely travel on a given route laid along a central passage between the article storage shelves arranged at both sides in the article storage chamber and can move vertically and laterally.
The article storage house of this type has an air supply unit and an air filter wherein air supplied from the air supply unit is cleaned by the air filter and it is blown from the rear portions of the article storage shelves toward the carry-in/out unit whereby dust is prevented from floating and staying on the article storage shelves.
Furthermore, since the carry-in/out unit has an elevator guiding unit for guiding the carry-in/out mechanism and an elevator driving unit which are mounted on a self-advancing carriage, dust is generated by abrasion between the elevator guide unit and the elevator driving unit and the self-advancing carriage. There occurs such a problem that the dust so generated is neither drawn toward the floor nor removed from the floor but moves toward the article storage shelves so that the dust adheres to the semiconductor wafer, etc. To prevent such problem, there are provided a cover for covering the entire of the elevator guiding unit and elevator driving unit mounted on the self-advancing carriage of the carry-in/out unit and an air drawing unit mounted on the lower surface of the self-advancing carriage for drawing the dust generated in the cover and removing the dust and discharging it toward the floor.
As mentioned above, there is formed a clean air current which flows from the rear portions of the article storage shelves toward the central passage between the article storage shelves disposed at both sides in the conventional article storage house in a clean room so that the dust is prevented from adhering to articles on the article storage shelves (semiconductor wafers accommodated in a wafer cassette).
However, in the conventional article storage house, there are following problems. That is, the article storage house should have a large-scale clean air supply system equipped with a high-performance filter or a circulation system since the article storage house is structured to keep the entire inside thereof to be in a highly clean air atmosphere. Furthermore, the inside of the article storage house should be structured and shaped so as to keep the entire thereof clean. Still furthermore, the components of the article storage house should meet the requirements as set forth above. Accordingly, the conventional article storage house becomes large-scale as a whole and costs high and is inferior in the maintenance thereof.
In case of a power failure, the clean air supply system or the circulation system stops in the operation thereof, which generates such problems that cleanliness of the entire article storage house is lowered and the maintenance of the system can not be made free since the contamination, which is caused by the trouble of the maintenance of the carry-in/out unit and other components, spreads over the entire article storage house.
As the semiconductor IC has progressed recently in its degree of high integration, there has occurred the problem of the formation of the oxide film caused by the natural oxidation of the semiconductor IC during the storage thereof.
FIGS. 28 and 29 appear in a thesis entitled "Super LSI Ultra Clean Technology Symposium" published Nov. 19, 1990. FIG. 28 shows the relation between the thickness of an oxide film and the time involved in the formation of the oxide film which is formed by the natural oxidation of the silicon semiconductor wafer. FIG. 29 shows the relation between the resistivity of the silicon semiconductor wafer and an air-exposure time when the silicon semiconductor wafer is exposed to the atmosphere so that the natural oxide film is formed thereon and is subjected to an epitaxial growth. According to this thesis, if the silicon semiconductor wafer is exposed to the atmosphere, the rate of growth of the oxide film is increased after the lapse of 100 to 200 minutes as illustrated in FIG. 28 and the resistivity is sharply increased after the lapse of about 50 minutes in case of non-cleaning state (in case of forming the natural oxide film).
In order to prevent the growth of the natural oxide film, the wafer should be moved or carried in an inert gas atmosphere (e.g. N.sub.2 gas or dry air). The inert gas atmosphere should have O.sub.2 concentration which is less than 10 ppm and H.sub.2 O concentration of which is less than 100 ppm.
Accordingly, the treatment of the semiconductor, which has been made conventionally in a specially designed clean room, has been made in a mechanical interface unit, which is provided in such clean room as disclosed in, e.g. Japanese Patent Laid-Open Publication No. 60-143623 or in the mechanical interface unit while the same unit is in the inert gas such as N.sub.2 gas etc. atmosphere.
However, there occurs such a problem that the natural oxide film is formed in the article storage house because the semiconductor wafer is stored normally during several times to several days in the article storage house.
To overcome this problem, the applicant filed the patent application for "a wafer storage house" under No. 3-9401 wherein a wafer storage chamber can be always filled with inert gas inside the wafer storage chamber so that the natural oxide film is restrained from growing on the surface of the semiconductor wafer. FIG. 23 shows this wafer storage house.
In FIG. 23, a wafer storage house 300 has a double-walled structure, i.e. an outer wall 311 and an inner wall 312 wherein a space 313 is defined between the outer and inner walls 311 and 312 and a wafer storage chamber 314 which is defined in a space surrounded by the inner wall 312. The wafer storage chamber 314 has a filter wall 314A at the ceiling thereof through which inert gas is filtered and a drawing wall 314B at the bottom thereof from which the inert gas is drawn. The outer wall 311 hat a discharge port 311B lower portion of the drawing wall 314B. The wafer storage chamber 314 has wafer storage shelves 315 and 316 at the right and left walls thereof and fluid passages 317 and 318 which are formed between the rear portions of the wafer storage shelves 315 and 316 and the inner wall 312. The wafer storage shelves 315 and 316 have respectively a plurality of sectioned shelves 319 and filters 320 which partition each sectioned shelve 319 and the fluid passage 317 or 318. The wafer storage shelves have spaces 321 and 322 at the bottom portions thereof and the spaces 321 and 322 communicate with the fluid passages 317 and 318. A circulation pump 323 is disposed in the space 321. A circulation pump, not shown, is disposed in the space 322. There are provided a first path box 330 at a first carry-in/out opening and a second path box 331 at a second carry-in/out opening. The first and second path boxes 330 and 331 have respectively filters 330A and 331A at the ceilings thereof and discharge ports 330B and 331B at the bottom walls thereof.
Designated at 340 is a gas storage tank (in this case N.sub.2 gas cylinder) for storing inert gas therein (in this case N.sub.2 gas) and 350 is a flow control unit. A first gas supply pipe 341 extends from the storage tank 340 to the ceiling of the article storage house 300 and opens into the space 313 between the outer and inner walls 311 and 312. Designated at 351 is a variable flow valve. A second gas supply pipe 342 extends from the storage tank 340 to the upper portion of the first path box 330. Designated at 352 is a first ON/OFF valve. A third gas supply pipe 343 extends from the storage tank 340 to the upper portion of the second path box 331. Designated at 353 is a second ON/OFF valve and 354, 355 and 356 are normally closed valves.
A first oxygen concentration meter 361 measures the oxygen concentration in the article storage house 300. A second oxygen concentration meter 362 measures the oxygen concentration in the first path box 330 and a third oxygen concentration meter 363 measures the oxygen concentration in the second path box 331.
A flow control unit 350 receives signals representing oxygen concentration measured by the first to third oxygen concentration meters 361, 362 and 363 and monitors such oxygen concentration to thereby control the degree of opening of the variable flow valve 351 in response the difference between the valve measured by the first oxygen concentration meter 361 and the reference value of the same. The flow control unit 350 opens the first and second ON/OFF valves until the values measured by sensors 362 and 363 reach prescribed values or during given time.
A stacker crane 370 travels on rails laid on the floor of the central passage. Designated at 381 are air blowing fans and 382 are air drawing fans.
In the wafer storage house, the wafer storage chamber can be always filled with inert gas so that the natural oxide film is restrained from growing on the surface of the semiconductor wafer. Furthermore, inert gas current, which directs from the rear portions of the sectioned shelves toward the stacker crane, is generated in the wafer storage chamber so that dust is prevented from adhering to the semiconductor wafer in the sectioned shelves. Air in the entire storage chamber can be replaced by inert gas, namely, the inert gas purge can be performed it, the entire storage chamber.
The applicant filed the application for the invention of the wafer storage house under No. 3-9404 in which sectioned shelves of the wafer storage house are structured to form small rooms in each of which the inert gas purge is performed. This is illustrated in FIG. 24.
In FIG. 24, a wafer storage house is provided with a wafer storage unit 400 having a plurality of box-shaped sectioned rooms 411 which are arranged lengthwise and crosswise and accommodate wafer cassettes therein as shown in FIG. 25. Each of the sectioned rooms 411 has a froth door 420 at the front opening side thereof (the side from which the cassette is taken in and out by the moving/loading unit). Each of the sectioned rooms 411 has a filter 412A which defines a fluid supply passage 413A between a left wall 411A and itself and a filter 412B which defines a fluid supply passage 413B between a right wall 411B and itself wherein both the filters 412A and 412B define a wafer accommodating space 414 therebetween from which the wafer cassettes are taken in and out.
A space 415 for use in piping is defined between the upper and lower sectioned rooms of the wafer storage unit 400 and extends from the left wall 411A to the right wall 411B thereof as shown in FIG. 26. There are provided in the space 415 a gas supply pipe 431A which penetrates the bottom wall 411C of the sectioned room 411 and opens into the fluid supply passage 413A and a gas discharge pipe 431B which penetrates the bottom wall 411C and opens into the fluid supply passage 413B. The gas supply pipe 431A is connected to an inert gas storage tank (such as an N.sub.2 gas cylinder, not shown) provided outside the wafer storage unit 400 by way of a valve and the gas discharge pipe 431B opens into the outside of the wafer storage house by way of a discharge valve. A seal member 416 is provided at the periphery of the front door 420 of each sectioned room 411.
As shown in FIG. 27, the front door 420 has shafts 421 and 422 which extend upward and downward from one side thereof in which the shaft 421 is received by a receiving member 417 projecting from the sectioned room 411 and the shaft 422 is connected to a motor 424 by a coupling 423. Four magnetic member pins 441 project from four corners of the front door 420 at the rear surface thereof. Each sectioned room 411 has electromagnetic solenoids 442 therein which attract the magnetic member pins 441. The magnetic member pins 441 and the electromagnetic solenoids 442 constitute automatic lock units. There is a play in the coupling 423.
In the arrangement of the wafer storage house, air in the wafer accommodation spaces 414 of all the sectioned rooms 411 should be replaced by N.sub.2 gas before the wafer storage units 400 are used. For this purpose, the motor 424 of the front door 420 is driven to close the front door 420 and to energize the electromagnetic solenoid 442 to thereby operate the lock unit. When the lock unit operates, the magnetic member pins 441 are attracted toward the rear wall side of the wafter accommodating space 414 so that the front door 420 is pressed strong against to contact the opening of the sectioned room 411 by way of the seal member 416, which results in closing the wafer accommodating space 411 airtightly.
Successively, the valve of the gas supply pipe 431A is full open and the discharge valve disposed at the discharge port of the gas discharge pipe 431B. Accordingly, N.sub.2 gas is supplied from the inert gas storage tank to the fluid supply passage 413A of the cassette accommodating space 414 by way of the gas supply pipe 431A. The N.sub.2 gas so supplied to the fluid supply passage 413A enters the wafer accommodating space 414 by way of the filter 412A so that the wafer accommodating space 414 is filled with the N.sub.2 gas. Air occupied the wafer accommodating space 414 passes through the filter 412B and expelled toward the fluid discharge passage 413B and then flows in the gas discharge pipe 431B and finally discharged outside the wafer storage house whereby air in the wafer accommodating space 414 is replaced by the N.sub.2 gas. All the valves are closed upon completion of the replacement of air by N.sub.2 gas.
Since the concentration of the N.sub.2 gas in the wafer accommodating space 414 lowers as time lapses, the N.sub.2 gas should be replenished. For this purpose, a flow control unit is provided for controlling the valve in the gas supply pipe 431A. The closing control of the valve may be made by a time control system where the valve is open during a predetermined time in every given time interval or a control system where the concentration of oxygen is monitored and the valve is open when the concentration of oxygen exceeds the prescribed value.
The wafer cassettes are taken in or out from the cassettes accommodating space 414 by unlocking the automatic lock unit of the specified sectioned room 411 and by driving the motor 424 of the front door 420 for thereby opening the front door 420.
In this case, since the wafer accommodating space 411 is in the inert gas atmosphere except the front door 420 is open, the oxide film is restrained from growing on the surface of the semiconductor wafer while the latter is stored in the wafer storage house.
There are such problems in the wafer storage house as disclosed in the Japanese Patent Application No. 3-9401 that firstly a person is likely to suffocate in the wafer storage chamber since the entire wafer storage chamber is in the inert gas atmosphere, secondly if there occurs the necessity of the operations in the wafer storage chamber, it is very troublesome that the person can not enter the chamber unless the gas in the chamber should be discharged and the concentration of oxygen in the chamber is measured and it is confirmed that the suffocation is not likely to occur, and thirdly there remains anxious for the likelihood of suffocation even in the clean room when the gas in the chamber is discharged if the clean room is poorly ventilated.
It takes a long time for the chamber to be filled with a highpure inert gas after the inert gas in the wafer storage chamber is once discharged, which generates the problem that the natural oxide film starts to grow on the surface of the semiconductor wafer while the semiconductor wafer is stored.
Since the highpure inert gas costs high and it is expelled every time the gas inside the entire storage house is discharged, there occurs the problem that the highpure inert gas is consumed much.
The wafer storage house as disclosed in Japanese Patent Application No. 3-9404 has no problem of suffocation. However, since the inert gas is circulated inside the article storage house, oxygen enters the sectioned room when the inert gas is purged again in case of opening the sectioned room so that the sectioned room is hardly to be maintained in the highpure inert gas atmosphere, which generates the problem that the structure for preventing the inert gas from discharging is very complex in case of opening the door of the sectioned room.